Electrical fuses that have a fusible element are used to prevent overcurrents in low voltage systems. In the event of an overcurrent, the fusible element melts at the narrowest point of a constricted area (the so-called "narrow point") and breaks the electrical connection. A fusible element of this type must satisfy two requirements. First, it must safely break short circuit currents with a very short reaction time. Second, it must also protect the electrical system components from prolonged periods of small overcurrent conditions. In other words, the fusible element must withstand current strengths that exceed a rated current strength by a specific amount for a limited period without causing the fusible element to trip (so-called slow-action breaking characteristic). For example, a fuse with a rated current strength of 16 amperes must be able to withstand 1.25 times this rated current strength (i.e., must withstand 20 amperes) for at least one hour without melting the fusible element.
The tripping of the fusible element during an overcurrent condition is achieved by virtue of the fact that a coating of solder is applied to the body of the fusible element. The body of the fusible element is generally made of copper (Cu) or silver (Ag). In response to an overcurrent condition when the temperature in the constricted area reaches the melting point of the solder, the solder melts and combines with the element material. Some of the element material mixes with the molten solder. The alloy coating thus produced has a higher electrical resistance than the element material. This means that the temperature of the fusible element can withstand is again decreased. This, in turn, speeds up the destruction of the fusible element.
This process is primarily effected by the constructive design of the fusible element and by the materials used. It has been found that solders that have good flow characteristics, for example pure tin (Sn 99.9), when they melt at the narrow point of the constricted area, flow uncontrollably beyond the narrow point and spread over the surface of the fusible element. This leads to poorly reproducible breaking characteristics in the event of an overcurrent, or to a failure of the fuse due to the lack of solder at the narrow point. To overcome this problem, solders used in conventional fusible elements contain additives, for example cadmium (Cd), to prevent their flowing onto the fusible element.